The invention relates to a circuit arrangement for operating a discharge lamp, provided with
input terminals for connection to a supply voltage source, PA1 rectifying means coupled to the input terminals for rectifying a low-frequency supply voltage with a frequency f delivered by the supply voltage source, PA1 capacitive means coupled to outputs of the rectifying means, PA1 a DC-AC converter coupled to the capacitive means for generating a lamp current which comprises a DC component and a high-frequency AC component, the amplitude of the high-frequency AC component being modulated with a low frequency which is equal to twice the frequency f, and PA1 means V for adjusting the power consumed by the discharge lamp. PA1 a branch comprising a series arrangement of a first switching element and a second switching element, PA1 a load branch shunting one of the switching elements and provided with terminals for accommodating the discharge lamp, PA1 a control circuit coupled to the switching elements for rendering said switching elements alternately conducting and non-conducting at a high frequency,
Such a circuit arrangement is known from British Patent GB 2,119,184. The known circuit arrangement is designed more in particular for operating a low-pressure mercury discharge lamp. The means V render it possible to adjust the luminous flux of the discharge lamp through adjustment of the power consumed by the discharge lamp. The DC component of the lamp current contributes to the suppression of striations. It was found, however, that striations can occur, also in dependence on the composition of the plasma of the discharge lamp, especially when the power consumed by the discharge lamp is set for a comparatively low value. Since said DC component forms part of the lamp current, it is possible to set the luminous flux of the discharge lamp for a lower value than would be possible if the lamp current were to comprise exclusively a high-frequency AC component. If it is desired to set the discharge lamp luminous flux for a very low value, however, it was found to be not impossible without further measures to suppress the striations by the mere exclusive addition of a DC component is added to the high-frequency AC component of the lamp current.